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    SubjectRe: [PATCH 10/10] jump label v11: add docs
    On Mon, Sep 20, 2010 at 06:28:00PM -0400, Steven Rostedt wrote:
    > On Fri, 2010-09-17 at 12:05 -0400, Mathieu Desnoyers wrote:
    > > * Jason Baron (jbaron@redhat.com) wrote:
    > > > Add jump label docs as: Documentation/jump-label.txt
    > > >
    > > > Signed-off-by: Jason Baron <jbaron@redhat.com>
    >
    > Jason,
    >
    > Could you post this patch again with any updates you want to take from
    > Mathieu. No need to post the entire series, I'm pulling in the rest.
    > Just this patch.
    >
    > Thanks,
    >
    > -- Steve
    >

    Hi Steve,

    here's a re-post of the documentation patch. I've incorporated Mathieu's
    as well as Matt's feedback. Note that I've dropped the
    '!CC_OPTIMIZE_FOR_SIZE' optimize for size paragraph.

    thanks,

    -Jason


    Add jump label docs as: Documentation/jump-label.txt

    Signed-off-by: Jason Baron <jbaron@redhat.com>
    ---
    Documentation/jump-label.txt | 142 ++++++++++++++++++++++++++++++++++++++++++
    1 files changed, 142 insertions(+), 0 deletions(-)
    create mode 100644 Documentation/jump-label.txt

    diff --git a/Documentation/jump-label.txt b/Documentation/jump-label.txt
    new file mode 100644
    index 0000000..d26df30
    --- /dev/null
    +++ b/Documentation/jump-label.txt
    @@ -0,0 +1,142 @@
    + Jump Label
    + ----------
    +
    +By: Jason Baron <jbaron@redhat.com>
    +
    +
    +1) Motivation
    +
    +
    +Currently, tracepoints are implemented using a conditional. The conditional
    +check requires checking a global variable for each tracepoint. Although
    +the overhead of this check is small, it increases when the memory cache comes
    +under pressure (memory cache lines for these global variables may be shared
    +with other memory accesses). As we increase the number of tracepoints in the
    +kernel this overhead may become more of an issue. In addition, tracepoints are
    +often dormant (disabled) and provide no direct kernel functionality. Thus, it
    +is highly desirable to reduce their impact as much as possible. Although
    +tracepoints are the original motivation for this work, other kernel code paths
    +should be able to make use of the jump label optimization.
    +
    +
    +2) Jump label description/usage
    +
    +
    +gcc (v4.5) adds a new 'asm goto' statement that allows branching to a label.
    +http://gcc.gnu.org/ml/gcc-patches/2009-07/msg01556.html
    +
    +Thus, this patch set introduces an architecture specific 'JUMP_LABEL()' macro as
    +follows (x86):
    +
    +# define JUMP_LABEL_INITIAL_NOP ".byte 0xe9 \n\t .long 0\n\t"
    +
    +# define JUMP_LABEL(key, label) \
    + do { \
    + asm goto("1:" \
    + JUMP_LABEL_INITIAL_NOP \
    + ".pushsection __jump_table, \"a\" \n\t"\
    + _ASM_PTR "1b, %l[" #label "], %c0 \n\t" \
    + ".popsection \n\t" \
    + : : "i" (key) : : label); \
    + } while (0)
    +
    +
    +For architectures that have not yet introduced jump label support it's simply:
    +
    +#define JUMP_LABEL(key, label) \
    + if (unlikely(*key)) \
    + goto label;
    +
    +which then can be used as:
    +
    + ....
    + JUMP_LABEL(key, trace_label);
    + printk("not doing tracing\n");
    + return;
    +trace_label:
    + printk("doing tracing: %d\n", file);
    + ....
    +
    +The 'key' argument is thus a pointer to a conditional argument that can be used
    +if the optimization is not enabled. Otherwise, this address serves as a unique
    +key to identify the particular instance of the jump label.
    +
    +Thus, when tracing is disabled, we simply have a no-op followed by a jump around
    +the dormant (disabled) tracing code. The 'JUMP_LABEL()' macro produces a
    +'jump_table', which has the following format:
    +
    +[instruction address] [jump target] [key]
    +
    +Thus, to enable a tracepoint, we simply patch the 'instruction address' with
    +a jump to the 'jump target.'
    +
    +The calls to enable and disable a jump label are: enable_jump_label(key) and
    +disable_jump_label(key).
    +
    +
    +3) Architecture interface
    +
    +
    +There are a few functions and macros that architectures must implement in order
    +to take advantage of this optimization. As previously mentioned, if there is no
    +architecture support, we simply fall back to a traditional, load, test, and
    +jump sequence.
    +
    +* add "HAVE_ARCH_JUMP_LABEL" to arch/<arch>/Kconfig to indicate support
    +
    +* #define JUMP_LABEL_NOP_SIZE, arch/x86/include/asm/jump_label.h
    +
    +* #define "JUMP_LABEL(key, label)", arch/x86/include/asm/jump_label.h
    +
    +* void arch_jump_label_transform(struct jump_entry *entry, enum jump_label_type type)
    + see: arch/x86/kernel/jump_label.c
    +
    +* void arch_jump_label_text_poke_early(jump_label_t addr)
    + see: arch/x86/kernel/jump_label.c
    +
    +* finally add a definition for "struct jump_entry".
    + see: arch/x86/include/asm/jump_label.h
    +
    +
    +4) Jump label analysis (x86)
    +
    +
    +I've tested the performance of using 'get_cycles()' calls around the
    +tracepoint call sites. For an Intel Core 2 Quad cpu (in cycles, averages):
    +
    + idle after tbench run
    + ---- ----------------
    +old code 32 88
    +new code 2 4
    +
    +
    +The performance improvement can be reproduced reliably on both Intel and AMD
    +hardware.
    +
    +In terms of code analysis, the current code for the disabled case is a 'cmpl'
    +followed by a 'je' around the tracepoint code. So:
    +
    +cmpl - 83 3d 0e 77 87 00 00 - 7 bytes
    +je - 74 3e - 2 bytes
    +
    +total of 9 instruction bytes.
    +
    +The new code is a 'nopl' followed by a 'jmp'. Thus:
    +
    +nopl - 0f 1f 44 00 00 - 5 bytes
    +jmp - eb 3e - 2 bytes
    +
    +total of 7 instruction bytes.
    +
    +So, the new code also accounts for 2 less bytes in the instruction cache per tracepoint.
    +
    +
    +5) Acknowledgements
    +
    +
    +Thanks to Roland McGrath and Richard Henderson for helping come up with the
    +initial 'asm goto' and jump label design.
    +
    +Thanks to Mathieu Desnoyers and H. Peter Anvin for calling attention to this
    +issue, and outlining the requirements of a solution. Mathieu also implemented a
    +solution in the form of the "Immediate Values" work.
    --
    1.7.1


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